Method of producing ion sensitive film for ion sensor

ABSTRACT

There is disclosed a method of easily producing a long-lived ion sensitive film having excellent durability and used in an ion sensor. The method starts with preparing a monomer mixture consisting chiefly of monomer units including a functional group and a second group of bonded atoms. The functional group has a function of identifying a certain chemical substance. The second group can become an active species that induces a polymerization or bridging reaction by being irradiated with an electron beam or radiation. Then, the monomer mixture is irradiated with the electron beam or radiation in a low energy range. Thus, the monomer mixture is polymerized.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method of producing an ionsensitive film used in an ion sensor (especially, an ion selective film)that is employed to measure the activity of ions within a liquidsolution.

[0003] 2. Description of Related Art

[0004] In recent years, ion sensors (especially, ion selectiveelectrodes) for medical applications have been vigorously used toquantify ions (e.g., sodium, potassium, and chlorine ions) dissolved ina liquid within a living organism, such as blood or urine.

[0005] An existing ion sensor is shown in FIG. 1 and comprises a sensor(electrode) body 1 and an ion sensitive film 4 fixed to the front-endsurface of the body 1. The inside of the body 1 is filled with aninternal electrolyte 3. An internal electrode 2 made of silver chlorideis immersed in this internal electrolyte.

[0006] Referring next to FIG. 2, this ion sensor, indicated by numeral5, is immersed in a sample solution 6 together with a salt bridge 7. Onthe other hand, a reference electrode 9 is immersed in an aqueoussolution 10 of saturated potassium chloride together with the saltbridge. The potential difference between both electrodes is read by anelectrometer 8. Using a reference sample having a known concentration asan object to be measured, the electromotive force is measured, and acalibration curve is created. The ion concentration of the sample underinvestigation is found by comparing the electromotive force produced bythe sample against the calibration curve.

[0007] The performance of such an ion sensor also depends on theperformance of the used ion sensitive film. Known ion sensitive filmsused for ion sensors include (a) a film produced by preparing a filmsupport material made of polyvinylchloride and mixing an ion sensitivematerial into the film support material together with a plasticizer, (b)an ion-exchange film, and (c) a “film-like substance” (disclosed inJapanese Patent No. 2504513) consisting of a polymer having a quaternaryammonium group having a certain structure.

[0008] However, the ion sensor using the ion sensitive film of the type(a) above has the disadvantage that the electrode life is short, becausethe ion sensitive material within the film gradually dissolves into theliquid solution.

[0009] The ion sensor using the ion sensitive film of the type (b) has ashort life because ionic groups are introduced into a polymer bycovalent bonding, the polymer forming a film. It has been pointed out,however, that the film is an ion-exchange film generally used forelectrolytic purposes and so if the film is used as an ion sensitivefilm in an ion sensor, the film is affected greatly by interfering ions.

[0010] In the ion sensitive film of the type (c) as disclosed in theabove-cited Japanese Patent No. 2504513, a straight-chain polymer isformed by polymerizing monomers having a quaternary ammonium group of acertain structure. Thus, the film shows excellent response to chlorineions and good waterproofness. The ion selective electrode has a longlife and can measure chlorine ions with high sensitivity stably.

[0011] The ion sensitive film of the type (c) is formed by normalheating radical solution polymerization using a radical polymerizationstarter and, therefore, limitations are placed on the degree ofpolymerization. In addition, molecular weights are broadly distributed.Therefore, it can be said that there remains room for improvements ofdurability and life. In addition, in such normal radical solutionpolymerization, it takes a long polymerization reaction time to increasethe reaction rate. Furthermore, unreacted monomers remain. Additionally,it is laborious to refine the reaction mixtures.

SUMMARY OF THE INVENTION

[0012] We have discovered that a long-lived ion sensitive film having ahigh degree of polymerization, excellent durability, and a narrow rangeof molecular weights can be formed by preparing a monomer mixture andpolymerizing the mixture by irradiating it with an electron beam orradiation in a low energy range. The monomer mixture consists chiefly ofmonomer units including a functional group having an identificationfunction and a second group of bonded atoms capable of becoming anactive species that induces a polymerization reaction by beingirradiated with an electron beam or radiation. Furthermore, we havediscovered that an ion sensitive film which is used in an ion sensor andwhich does not need to be refined can be obtained directly from afilm-like monomer mixture, the monomer mixture reaching a high reactionrate in a quite short reaction time.

[0013] Other objects and features of the invention will appear in thecourse of the description thereof, which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a cross-sectional view of a general ion sensor;

[0015]FIG. 2 is a schematic diagram of a general ion-measuringinstrument;

[0016]FIG. 3 is a schematic diagram of an electron beam irradiationsystem used in Examples 1 and 2 of the present invention;

[0017]FIG. 4 is a chart illustrating the results of small-anglescattering measurements of synchrotron radiation emitted from a polymerfilm P1 produced in Example 1 of the present invention; and

[0018]FIG. 5 is a chart illustrating the results of small-anglescattering measurements of synchrotron radiation emitted from a polymerfilm P2 produced in Example 1 of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] Monomer units used in the present invention have a functionalgroup having a function of identifying a certain chemical substance. Nolimitations are imposed on this functional group, as long as thefunctional group can bond, directly or via a necessary connecting group,to a second group, such as vinyl group or allyl group, that can becomean active species inducing a polymerization or bridging reaction bybeing irradiated with an electron beam or radiation.

[0020] Examples of such a functional group include functional groupshaving crown ether structure, valinomycin structure, calixarenestructure, porphyrin structure, organotin structure, organomercuricstructure, and cyclic polyamine structure, anion-exchange groups such asquaternary ammonium salts, and cation-exchange groups, such as sulfonicgroup and carboxyl group.

[0021] Examples of the certain chemical substance that can be identifiedby these functional groups include cations (such as sodium ions,potassium ions, lithium ions, calcium ions, magnesium ions, cesium ions,strontium ions, copper ions, cadmium ions, lead ions, thallium ions, andsilver ions) and anions (such as fluorine ions, chlorine ions, iodineions, thiocyanate ions, nitrite ions, monohydrogen phosphate ions,dihydrogen phosphate ions, perchlorate ions, nitrate ions, bromate ions,carbonate ions, acetate ions, and sulfate ions).

[0022] Where monomers having a self-organizing function are used, themonomers in an arrayed state are irradiated with an electron beam, thusinducing polymerization. This can enhance the selectivity with respectto a certain kind of ions. In order to enhance the orientation of afunctional group, it may be necessary to insert a different monomer intothe main chain of a polymer, for providing a distance at which a pendantfunctional group orients itself easily.

[0023] The monomer units, according to the present invention, can beoligomers if they contain unsaturated groups capable of becoming activespecies that induce a polymerization or bridging reaction whenirradiated with an electron beam or radiation.

[0024] This electron beam-induced polymerization of monomers or monomermixtures can take various forms, such as bulk polymerization andsolution polymerization. For ease of operability of later process steps,it is desired to irradiate a liquid spread like a film 17 on a substratewith an electron beam. In the case of a liquid monomer, it is usedintact. In the case of a solid monomer, it is necessary to liquefy it byadding a solvent or monomer necessary for the liquefaction or byheating. Preferably, the thickness of the liquid monomer or monomermixture over the substrate is about 30 to 500 μm. No limitations areplaced on the material of the substrate 18, as long as it can hold themonomer thereon. For example, the material of the substrate is glass,polyethylene, paraffin, or silicon. If possible, the monomer may befloated over water, oil, or the like.

[0025] An electron beam irradiation system is schematically shown inFIG. 3. This system includes an electron gun 12 placed within a vacuum.Electrons extracted from the gun 12 as an electron beam 15 areaccelerated by an accelerating tube 13 and secondary scanning coils 14and penetrated through titanium foil 16. Then, the beam is shot at atarget 17 placed within a vacuum or ambient of an inert gas, such asnitrogen gas.

[0026] The electron beam is shot at a low accelerating voltage of lessthan 1,000 kV, more preferably, less than 300 kV. The dose of theelectron beam is preferably 50 to 500 kGy, although the value variesdepending on the treated monomer. With respect to the actual electronbeam irradiation time, for the same dose of electron beam, the electronbeam is shot at a relatively low energy for a long time in some casesand shot at a relatively high energy for a short time in other cases. Toprevent production of excessive active species, such as radicals, theformer method is desirable to obtain large molecular weights of uniformmolecular weight distribution.

[0027] To polymerize monomers or monomer mixture on the substrate whilepreventing decomposition, the monomers are preferably kept cool even byforced cooling. Usually, the monomers are kept below room temperature.In the case of monomers giving rise to low reaction rates, they may beheated mildly to promote the reaction. In this case, the heatingtemperature is preferably below 50° C.

[0028] The monomers or monomer mixture on the substrate must be placedwithin a vacuum or ambient of an inert gas, such as nitrogen gas. Thepresence of oxygen will produce a radical source, which is undesirablefor the electron beam-induced polymerization of the present invention.

[0029] The following nonlimiting examples are provided to furtherillustrate the present invention.

EXAMPLE 1

[0030] Substituted styrene monomers having pendant functional groupsconsisting of quaternary ammonium salts represented by Formulas 1 and 2,respectively, were synthesized by a known method.

[0031] Then, 50 μl of each type of the synthesized liquid monomers wasdripped onto the substrate by a micropipette. The dripped monomer wasspread to a thickness of about 100 μm on the substrate. The monomer wasirradiated with an electron beam by an electron beam irradiation systemas shown in FIG. 3 in an ambient of nitrogen.

[0032] The energy conditions included an accelerating voltage of 150 kVand a filament current of 10 mA. The electron-beam dose per exposure was100 kGy. The number of exposures was varied to vary the electron-beamdose to 100, 200, and 300 kGy. In this way, the state of each filmformed on the substrate was examined. The results are given in Table 1.TABLE 1 Monomer Formula 1 Formula 2 electron-beam dose (kgr) 100 200 300100 200 300 substrate glass x · · x · · polyethylene x · · x x ·paraffin x · · x x · n-paraffin x · · x x · silicon grease X · · x x ·

[0033] Films could be almost completely formed on every substrate withan electron-beam dose greater than 300 kGy. The total actual exposuretime at electron-beam dose of 300 kGy was about 15 seconds.

[0034] The monomers represented by Formulas 1 and 2, respectively, wereexposed on glass substrates with an electron beam at a dose of 300 kGy,producing solid polymer films P1 and P2, respectively.

[0035] The obtained solid polymer films P1 and P2 were subjected todissolution testing. That is, they were allowed to stand at roomtemperature for 4 days. The results are shown in Table 2. TABLE 2solvent P1 P2 benzene · · toluene · · n-hexane x x n-heptane x xmethanol x x ethanol x x acetone · · methyl ethyl ketone x x diethylether x x tetrahydrofuran · · dioxane x x ethyl acetate x x methylenechloride x x chloroform x x dimethyl formaldehyde · · dimethylacetamide· · dimethylsulfoxide · · carbon tetrachloride x x acetonitrile x x

[0036] The results of the dissolution tests show that the films wereinsoluble to many solvents but the films swelled in some solvents. Thefilms were dissolved in none of these solvents. It is estimated that thefilms are linear polymers which are either highly polymerized orconsiderably cross-linked.

[0037] The materials represented by Formulas 1 and 2 are monomers havinga self-organizing function. Small-angle scattering measurements ofsynchrotron radiation were performed on the films P1 and P2. Diffractionpeaks were observed. Thus, it can be seen that the monomers have anarrayed structure (see FIGS. 4 and 5). The plane spacing of each ofthese polymers was calculated from these peaks. It has been found thateach of these polymers consists of two orientations of molecules.

[0038] These solid polymer films P1 and P2 were attached to the frontend of the electrode body shown in FIG. 1, and potentiometricmeasurements were performed with the measuring instrument shown in FIG.2. Aqueous solutions of sodium chloride of 10⁻¹ M and 10⁻³ M,respectively, were used as sample liquids in the measurements. Apotential gradient of about −58 mV/decade was observed for both P1 andP2. This value agrees well with a calculated value of −59 mV/decadefound from the Nernst equation. Consequently, it is seen that the ionsensitive film has a sufficient sensitivity for an ion sensor.

EXAMPLE 2

[0039] Placed on a glass substrate was 2-allyloxymethyl-18-crown 6-ether(produced by Tokyo Kasei Kogyo Co., Ltd.) represented by Formula 3. Thismaterial was irradiated with an electron beam at an accelerating voltageof 150 kV, a filament current of 10 mA, and an electron-beam dose of 300kGy, in the same way as in Example 1. In this manner, a solid polymerfilm P3 was prepared.

[0040] The obtained solid polymer film P3 was subjected to dissolutiontests. That is, it was allowed to stand at room temperature for 4 days.The results are shown in Table 3. TABLE 3 solvent P3 benzene · toluene ·n-hexane x n-heptane x methanol x ethanol x acetone · methyl ethylketone x diethyl ether x tetrahydrofuran · dioxane x ethyl acetate xmethylene chloride x chloroform x dimethyl · formaldehydedimethylacetamide · dimethylsulfoxide · carbon tetrachloride xacetonitrile x

[0041] The results of dissolution tests show that the polymers exhibiteddissolution behaviors in the presence of various solvents, in exactlythe same way as in Example 1. Again, it is estimated that the films arelinear polymers which are either highly polymerized or considerablycross-linked.

[0042] In the same way as in Example 1, the solid polymer film P3 wasattached to the front end of the electrode body, and potentiometricmeasurements were carried out. Aqueous solutions of sodium chloride of10⁻¹ M and 10⁻³ M, respectively, were used as sample liquids in themeasurements. A potential gradient of about −55 mV/decade was observed.It is observed that the film also has a sufficient sensitivity for anion sensor.

[0043] As described thus far, the present invention makes it possible toproduce a long-lived ion sensitive film which is used in an ion sensorand has excellent durability, a high degree of polymerization, and anarrow range of molecular weights, by preparing a monomer mixtureconsisting chiefly of monomer units including a functional group havingan identification function and a second group of bonded atoms capable ofbecoming an active species and by irradiating the monomer mixture withan electron beam or radiation in a low energy range to polymerize themonomer mixture. The active species induces a polymerization or bridgingreaction by being irradiated with the electron beam or radiation.

[0044] Furthermore, the invention provides an ion sensitive film whichis used in an ion sensor and can be directly produced from a film-likemonomer or mixture without requiring refining, the film-like monomerreaching a high reaction rate in a quite short reaction time.

[0045] Additionally, in a monomer having a self-organizing function, themonomer can be polymerized in an arrayed state. This enhances theselectivity with respect to the ions to be identified.

[0046] Having thus described our invention with the detail andparticularity required by the Patent Laws, what is desired protected byLetters Patent is set forth in the following claims.

The invention claimed is:
 1. A method of producing an ion sensitive filmfor an ion sensor, said method comprising the steps of: preparing amonomer mixture consisting chiefly of monomer units including afunctional group having a function of identifying a certain chemicalsubstance and a second group of bonded atoms capable of becoming anactive species that induces one of a polymerization reaction and abridging reaction by being irradiated with an electron beam orradiation; and irradiating said monomer mixture by the electron beam orradiation to polymerize the monomer mixture.
 2. The method of claim 1,wherein said monomer units have a self-organizing function and induce apolymerization reaction by being irradiated with the electron beam,whereby the monomer units are polymerized.
 3. The method of claims 1 and2, wherein said monomer units are maintained at a low temperature. 4.The method of claim 3, wherein said low temperature is below roomtemperature.
 5. The method of claims 1 and 2, wherein said monomer unitsreact slowly, and wherein said monomer units are heated to a giventemperature.
 6. The method of claim 5, wherein said given temperature isbelow 50° C.
 7. The method of claim 1, wherein said functional group isone or more selected from the group consisting of functional groupshaving crown ether structure, valinomycin structure, calixarenestructure, porphyrin structure, organotin structure, organomercuricstructure, and cyclic polyamine structure, and ion-exchange groups. 8.The method of claim 1, wherein said certain chemical substance is an ionspecies.
 9. The method of claim 8, wherein said ion species is one ormore species selected from the group consisting of cations includingsodium ions, potassium ions, lithium ions, calcium ions, magnesium ions,cesium ions, strontium ions, copper ions, cadmium ions, lead ions,thallium ions, and silver ions.
 10. The method of claim 8, wherein saidion species is one or more species selected from the group consisting ofanions including fluorine ions, chlorine ions, iodine ions, thiocyanateions, nitrite ions, monohydrogen phosphate ions, dihydrogen phosphateions, perchlorate ions, nitrate ions, bromate ions, carbonate ions,acetate ions, and sulfate ions.
 11. The method of claim 1, wherein saidelectron beam or radiation is shot in a vacuum or ambient of an inertgas.
 12. The method of claim 1, wherein said monomer mixture is spreadover a substrate and irradiated with said electron beam or radiation.13. The method of claim 12, wherein said monomer mixture spread over thesubstrate has a thickness in the range of from 30 to 500 μm.
 14. Themethod of claim 1, wherein said irradiating electron beam is acceleratedby an accelerating voltage of less than 1,000 kV.
 15. The method ofclaim 1, wherein the dose of said electron beam is in the range of from50 to 500 kGy.